Wastewater examined as way to track cannabis use

Statistics Canada is experimenting with an innovative technique in an effort to more accurately measure cannabis use in Canada.

The government agency is exploring the use of wastewater-based epidemiology (WBE) to produce statistics on cannabis use before and after the drug was made legal across the country in October.

One of the challenges in collecting statistics on cannabis is that there is a risk that people may under-report their consumption, at least initially, in surveys conducted by the agency. Stigma associated with use and the reluctance to disclose purchases from non-regulated suppliers are two factors that could contribute to under-reporting.

One consequence of under-reporting is that the size of the black market for cannabis will be similarly underestimated.

When people consume THC, the main psycho-active constituent of cannabis, their bodies process it into metabolites that are later eliminated from the body and typically flushed into the municipal wastewater system. Through direct analysis of the concentrations of these metabolites in wastewater, Statistics Canada can estimate the total mass of cannabis consumed.

WBE is ideal because it is cost-effective, does not impose a response burden on Canadians or businesses, yields relatively rapid results and can be used on smaller areas, such as a city.

It may also be used to estimate the size of the illegal cannabis market following legalization, by allowing Statistics Canada to subtract the volume of legal cannabis retail sales from total consumption as measured through WBE.

The pilot project, which ran from March to August, covered the catchment areas served by 15 wastewater treatment plants in five large urban centres across the country, representing nearly 8.4 million Canadians.

Statistics Canada is still exploring the benefits and limits of using WBE to produce statistics on drug use. Subsequent tests and results will allow the agency to better assess the long-term feasibility of using wastewater as an official data source.

In December 2017, Statistics Canada released its first estimates of cannabis consumption ― not based on WBE ― according to which Canadians had consumed 773 tonnes of cannabis in 2017. At the time of publication, the agency acknowledged that the figure may have been under-reported and that it should therefore be used as the lower limit of an estimated range.